The German Patent Application DE 195 26 434 A1 describes a method for reversibly storing hydrogen which provides for using sodium alanate, potassium alanate, sodium-lithium alanate, sodium-potassium alanate or lithium-potassium alanate as reversible hydrogen storage materials.
Using natrium alanate NaAlH4 as an example, it describes reversibly storing hydrogen in accordance with the following multi-stage procedure:NaAlH4⅓Na3AlH6+⅔Al+H2  (1)⅓Na3AlH6NaH+⅓Al+½H2  (2)NaHNa+½H2  (3)
In this context, the hydrogen exchange of the stages is performed in accordance with equations (1) and (2) at temperatures of around 100° C. and pressures of several MPa, which is especially significant for low-temperature fuel cells. Of the altogether 7.6% by weight of H in the compound NaAlH4, 5.6% by weight of H can be theoretically exchanged in accordance with equations (1) and (2). However, if the assumption is that pure NaAlH4 is used, the reaction times last several days, since kinetic barriers delay the conversion of the material.
Therefore, in accordance with the German Patent Application DE 195 26 434 A1, the addition of a catalytically active dopant, typically in quantities of 0.2 to 10 mole % relative to the alkali metal alanate, is a precondition for moderate operating pressures and temperatures. Advantageously suited for this are compounds of transition metals from the 3rd to 5th group of the periodic system (Sc, Y, Ti, Zr, Hf, V, Nb, Ta) of iron, nickel or of a rare earth metal, preferably alcoholates, halogenides, hydrides, organometallic or intermetallic compounds.
The German Patent Application DE 101 63 697 A1 describes hydrogen storage materials made of the above-mentioned alanates or of mixtures of aluminum metal with alkali metals and/or alkali metal hydrides, which are doped with metal catalysts having particle sizes of 0.5 to 1,000 nm and specific surface areas of 50 to 1,000 m2/g, for which transition metals of groups 3 through 11 of the periodic system or aluminum, as well as alloys, mixtures or compounds of these metals, in particular titanium, titanium-iron and titanium-aluminum are used.
B. Bogdanovic, M. Schwickardi, Journal of Alloys and Compounds 253-254 (1997), p. 1, and D. L. Anton, Journal of Alloys and Compounds 356-357 (2003) pp. 400-404, ascertained that, as a dopant, titanium has the best properties in terms of an acceleration of the hydrogen exchange reaction. J. Wang, A. D. Ebner, R. Zidan, J. A. Ritter, Journal of Alloys and Compounds 391 (2005) pp. 245-255, likewise demonstrated that a doping process employing a mixture of different transition metals, in particular Ti with Zr, Fe or a mixture therefrom, is advantageous.
At the present time, a wet impregnation process is used as a doping process in which solvent is added under agitation of a transition metal compound, or a solventless doping process is used in accordance with U.S. Pat. No. 6,471,935 which provides for the hydrogen carrier material to be mechanically alloyed with the transition metal compound in a ball mill. In both cases, the higher-valency transition metal chemically reacts with the alanate to form the reduced metal. Depending on the type and added quantity of catalyst precursor, a certain amount of metal hydride is consumed during the course of the reaction (oxidized). Besides finely dispersed and catalytically active Ti0, either gaseous organic by-products are produced that can damage the fuel cell, or, as expressed by the equation(1−x) NaAlH4+x TiCl3→(1−4x) NaAlH4+3x NaCl+x Ti0+3x Al0+6x H2  (4),solids, such as Al0 and NaCl, form, which do not store any hydrogen and, therefore, degrade the gravimetric storage capacity of the material.
E. H. Majzoub and K. J. Gross, Journal of Alloys and Compounds (2003), 356-357, p. 363 and P. Wang and C. M. Jensen, Journal of Alloys and Compounds (2004), 379, pp. 99-102 attempted to overcome this disadvantage by dispensing with expensive TiCl3 and by using finely dispersed metallic Ti or a cubic TiAl3 alloy instead as dopant for NaAlH4. However, the production required very long ball milling times and, at a charging time of 12 h, for example, a working temperature of 120° C., and an H2 charging pressure of 12 Mpa, the material produced in this manner exhibited only very slow kinetics.
Since high costs are entailed in the chemical production of alanate, the German Patent Application DE 100 12 794 A1 describes producing the alanate by performing the reverse reaction, as expressed by equations (2) and (1), i.e., using inexpensive starting materials, such as NaH and Al, as well as transition metal catalysts or rare earth metal catalysts as dopants. However, if TiCl3 is used as a precursor for the catalyst at a price of approximately 15-20 euro/g, this leads to catalyst costs of around 50,000 euro for 100 kg of storage material, which considerably limits commercial use.
It would be more economical and practical to use the far less expensive TiCl4 at a price of approximately 0.02 euro/g. The price for an equivalent Ti quantity would only amount to around 50 euro for 100 kg of storage material. However, this compound is tetravalent and only contains about 25% by weight of Ti; the remainder is inactive chloride, so the result is a lower gravimetric storage capacity of the material.
When a metal hydride is doped with a multivalent Ti compound such as TiCl4, a portion of the hydrogen storage material is consumed, metallic Ti0 forming as the result of a redox reaction. In the process, besides metallic Al0, which does not store any hydrogen, other secondary products having inactive storage capacity are formed, such as NaCl in particular, as expressed by equation (4).
Also, when the storage material is produced by performing the reverse reaction of equation (2), material having inactive storage capacity is formed after adding the catalyst Ti precursor as expressed by4NaH+TiCl4→4NaCl+Ti+2H2  (5).
TABLE 1Theoretical proportion of material having inactive storagecapacity when Ti, TiCl3 and TiCl4 are usedas catalyst precursorProduction method inProportion of materialaccordance withPrecursorhaving inactivethe equation[2 mole %]storage capability(4) NaAlH4 + precursorTiCl311.3% by weight(4) NaAlH4 + precursorTiCl414.4% by weight(5) NaH + Al + precursorTiCl3 8.8% by weight(5) NaH + Al + precursorTiCl411.1% by weight— NaH + Al + Ti— 1.9% by weight
According to Table 1, when TiCl3 or TiCl4 is used as a catalyst precursor, respectively metallic Ti, as the case may be at a concentration of 2 mole %, the theoretical proportion of material having inactive storage capacity is between 1.9 and 14.4% by weight, depending on the preparation method. Thus, the quantity of inactive storage material produced by using inexpensive TiCl4 would be approximately 6-8 times that produced using metallic Ti, which yields by far the smallest proportion of substances having inactive storage capacity. However, as already described above, due to the long ball milling times during production and the slow kinetics of the product, a use of metallic Ti powder is not beneficial.